Force FileVault to use 256-bit key encryption

Solution 1:

Most documentation I'm finding seems to indicate that the key's used in macOS 10.9 and higher use a 256 bit key by default.

Encryption and authentication support

FileVault 2 uses the Advanced Encryption Standard (AES) encryption algorithm, which delivers robust protection for stored data. Until mid-2013, it only supported the use of 128-bit keys, not 256-bit keys. Although 128-bit keys are technically acceptable in many environments, organizations are rapidly moving toward 256-bit keys to thwart emerging threats.

The latest versions of Mac OS X, starting with 10.9, support 256-bit AES keys, so organizations wishing to enable FileVault 2 on legacy systems should be cautious about the 128-bit key strength present in older Mac OS X versions, Lion (10.7) and Mountain Lion (10.8).

Reference: Apple FileVault 2: Full disk encryption software overview

This PDF and the Wikipedia page seem to indicate this as well.

• macOS Security - Overview for IT 2018
• Wikipedia - FileVault